1. Field of the Invention
The present invention relates to a thin film type monolithic semiconductor device which has a plurality of thin film transistors (TFTs). The TFTs fabricated in the present invention are formed both on insulating substrates such as glass or the like and on semiconductor substrates such as single crystal silicon or the like. More particularly, the present invention relates to a semiconductor circuit which has a low speed operating matrix circuit such as a monolithic active matrix type circuit (which is used in a liquid crystal display or the like) and a high speed operating peripheral circuit to drive the matrix circuit.
2. Description of the Prior Art
In recent years, research has been made on semiconductor devices of insulated gate type, the devices having a thin film-like active layer (which is referred to as an active layer) on an insulating substrate. In particular, efforts are concentrated on studying thin film gate type transistors, or so-called thin film transistors (TFTs). The TFTs are formed on a transparent insulating film, and are used for the control of each pixel and in a driving circuit in a display which is formed of a liquid crystal or the like and which has a matrix structure.
Examples of thin film semiconductors which constitute TFTs include amorphous silicon semiconductors and crystalline semiconductors which are crystallized by heating or laser light irradiation of the amorphous silicon semiconductors. The TFTs using these amorphous silicon thin film and crystalline silicon thin film are referred to as amorphous silicon TFTs and crystalline TFTs.
Generally, the field mobility of semiconductors in an amorphous state is small, and therefore cannot be used in TFTs which are required to be operated at a high speed. Therefore, research and development has been carried out in recent years on crystalline TFTs for the fabrication of circuits which have higher performance.
Crystalline semiconductors have large field mobilities, and therefore can be operated at high speed. Since NMOS TFTs and PMOS TFTs are obtained with the crystalline silicon in the same manner, a CMOS circuit can be formed. For example, in an active matrix type liquid crystal display device, display devices with a monolithic structure in which both an active matrix type part and a peripheral circuit (drivers or the like) are constituted with a CMOS crystalline TFTs are known.
FIG. 3 shows a block diagram of a monolithic active matrix circuit used in a liquid crystal display. In a structure shown in FIG. 3, a column decoder 1 and a line decoder 2 are provided on a substrate 7 as a peripheral driver circuit. Further, in a matrix area 3 in which a plurality of pixels are arranged in a matrix configuration, a plurality of pixel circuits 4 which comprise transistors and capacitors are formed so that the matrix area and the peripheral circuit are connected with wires 5 and 6. The TFTs used in the peripheral circuit are required to be operated at a high speed while the TFTs used in the pixel circuit are required to have low current characteristics. These two characteristics are inconsistent to each other in terms of physics. However, the TFTs used in the peripheral circuit and the TFTs used in the pixel circuit are demanded to be formed on the same substrate at the same time.
However, the TFTs fabricated in the same process all exhibit the same characteristics. For example, means of crystallization (so-called laser anneal) can be used to obtain a crystalline silicon. However, in a silicon which has been crystallized by laser crystallization, the TFTs in the matrix area and TFTs in the peripheral driving circuit area exhibit the same characteristics. Therefore, the low leak current characteristics demanded of the pixel circuit and the high field mobility characteristics demanded of the peripheral circuit can coexist with great difficulty. The present invention is intended to solve such a difficult problem.
As a result of the investigations by the present inventors, it has been made clear that the crystallization of the silicon is promoted by doping an extremely small amount of a metal material to a silicon film in a substantially amorphous silicon so that the temperature of crystallization can be lowered, and the time required of the crystallization is shortened. As a catalyzing material, one or more kinds of elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, Ag and Au and further a compound of these elements (for example, a silicide) can be used.
Specifically, films, powders, clusters or the like containing these metal elements are allowed to adhere to the amorphous silicon film. Otherwise, these catalyzing elements are introduced into the amorphous silicon film by a method of ion doping process or the like followed by subjecting the film to the heat treatment at 550xc2x0 C. or lower about 4 hours.
Quite naturally, there is a relation such that the crystallization time is shorter with higher annealing temperature. In addition, there is also a relation such that the crystallization temperature is lower, and the crystallization time is shorter with a higher concentration of the metal element. The investigation of the present inventors has revealed that the concentration of at least one element out of the aforementioned elements is required to be set to 1xc3x971016 cmxe2x88x923 or more to carry out the crystallization in a manner of thermal equilibrium. Further, it has been also made clear that when the concentration becomes 5xc3x971019 cmxe2x88x923 or more, the physical characteristics as a semiconductor material are lost. Thus, the metal element concentration to accelerate the crystallization of silicon is preferably within a range of 1xc3x971016 cmxe2x88x923 to 5xc3x971019 cmxe2x88x923. Further, it has been also made clear that use of nickel out of the aforementioned metal elements is the most favorable. Incidentally, the concentration of the impurity in this specification is defined as the minimum value measured with the SIMS (second ion mass spectrometer).
Further, it has been also made clear that a domain (which is referred to as a mono-domain area) with a large grain diameter is obtained by heating a sample at 450xc2x0 C. or higher at the time of the laser light irradiation in a method for obtaining a crystalline silicon thin film by carrying out the crystallization by irradiating an amorphous silicon film with laser light. This mono-domain area has a crystal structure inside of which can be regarded as a substantially single crystal.
No crystal grain boundary exists inside of the mono-domain area. In addition, the mono-domain has point defects that should be neutralized unlike the single crystal silicon wafer. The mono-domain contains 1xc3x971016 cmxe2x88x923 to 1xc3x971020 cmxe2x88x923 of a recombination center neutralizer such as hydrogen or a halogen element which neutralizes the point defect.
In the case where a metal element such as the aforementioned nickel or the like is introduced into a starting film for forming the aforementioned mono-domain area, a mono-domain area with smaller defect concentration can be obtained. In the case where a thin film transistor is fabricated by using the mono-domain area which is formed by the introduction of this metal element, it is possible to obtain a TFT which has a high field mobility and allows the passage of a larger ON current. In other words, it is possible to obtain a TFT which has characteristics required for arranging the TFTs in a peripheral circuit area of a liquid crystal display with an active matrix structure.
Further, it has been made clear that an amorphous silicon TFT can be sufficient as the TFT which is arranged in each pixel in a matrix area because of the problem of the response speed of the liquid crystal (even when the thin film transistor is operated at any high speed, the liquid crystal cannot follow the speed). Since the OFF current is small instead of the fact that the TFT cannot be operated at a high speed, the amorphous silicon TFT has the most appropriated characteristics for switching the pixel.
The present invention is characterized by the fabrication of a thin TFT having a selectively different characteristics on the same substrate by using the operation of a metal element which promotes the crystallization of the aforementioned silicon. In other words, an amorphous silicon film is formed, a material selectively having a catalyst element on part thereof is closely contacted or mixed, and then a required area is irradiated with laser light or strong light having the same intensity or laser light or strong light having the same intensity is selectively scanned in a state in which the sample is heated at 450 to 750xc2x0 C., or preferably at 450 to 600xc2x0 C. so that an area where a thin film transistor constituting a peripheral circuit area constitutes a mono-domain area thereby forming a TFT with a high field mobility, a high speed operation and a structure that allows a large ON current to flow by using the aforementioned area. Then, the matrix area is retained in an amorphous state, and an amorphous silicon TFT for switching pixels is formed by using the area.
In this manner, a mono-domain TFT which can be operated at a high speed and an amorphous silicon TFT which has a low OFF current characteristics can be selectively fabricated on the same substrate.
Incidentally, it is very important to heat the sample at 450 to 750xc2x0 C. or at 450 to 600xc2x0 C. in consideration of the heat resistance of the glass substrate at the time of irradiating the sample with the laser light or strong light for forming a mono-domain area.
Further, it is effective to heat treat the sample before or after the irradiation of the sample with laser light or strong light for forming a mono-domain area. In the case where the sample is heat treated before the irradiation of laser light, a nucleus of a crystal growth at the time of laser light irradiation can be formed. Further, when the sample is heat treated after the irradiation of laser light, defects in the film can be reduced. Further, heat treating the sample before and after the laser light irradiation provides the following two effects. That is, the crystal nucleus can be formed and the defects in the film can be reduced.
According to a main aspect of the present invention, there is provided a monolithic active matrix circuit which is formed on a substrate, characterized in that a metal element promoting the crystallization of silicon is doped at a concentration of 1xc3x971018 to 5xc3x971019 cmxe2x88x923 into at least a part of an active area of a thin film transistor which constitutes a peripheral circuit, the active area of the thin film transistor in a matrix area can be constituted of an amorphous silicon semiconductor film, and a channel formation area in at least a part of the TFT which constitutes the aforementioned peripheral driving circuit.
In the aforementioned structure, a structure shown in FIG. 3 can be formed as xe2x80x9ca monolithic active matrix circuit which is formed on the substratexe2x80x9d. Further, as xe2x80x9cat least part of the TFT which constitutes the peripheral driving circuitxe2x80x9d, a TFT which constitutes a peripheral driving circuits 1 and 2 shown in FIG. 3 can be formed. Further, examples of the active area of the TFT include a source area and a drain area of a TFT and an area which includes a channel formation area as shown in FIG. 1(c) 142 and 143. In this active area, an offset gate area and a light dope area may be included.
Further, examples of the xe2x80x9cmatrix areaxe2x80x9d include an area denoted by reference numeral 3 in FIG. 3. This matrix area is an area where a plurality of pixels (which amount to several million in number) are arranged. Further, examples of the xe2x80x9cstructure constituted of a thin film silicon semiconductor film in which a channel formation area has a mono-domain structurexe2x80x9d, include an example shown in FIGS. 1B and 1C.
That is, active regions 141 and 142 of the TFTs are formed in mono-domain areas 121 and 122. Further, reference numeral 123 denotes an amorphous silicon semiconductor film. An active layer 143 of the amorphous silicon TFT which is arranged in a matrix area is formed by using this area.
Also, not all the TFTs arranged in the peripheral driving circuits are needed to have a structure that allows a high field mobility and a high speed operation and which allows a large ON current to flow. When an inverter circuit as shown in FIG. 5A or 5B is used, the N-channel TFTs 601 and 603 function as a negative load resistance, which does not need a high field mobility, a high speed operation or a large ON current to flow.
FIG. 5A shows a basic structure of an inverter in which a depression type transistor is used as an N-type TFT 601 which functions as a load and an enhancement type transistor is used as an N-type TFT 602. In addition, FIG. 5B shows a basic structure of an inverter in the case where an enhancement type transistor is used as an N-type TFT 603 which functions as a load, and an enhancement type transistor is also used as an N-type thin film transistor 602.
In such a case, it is not always necessary to constitute the active areas of the thin film transistors 601 and 603 by using a metal element which promotes the crystallization. Further, the active areas of the TFTs 601 and 603 need not be constituted in a mono-domain structure.
Therefore, in such a case, TFTs denoted by reference numerals 602 and 604 in FIGS. 5A and 5B correspond to TFTs which constitute the peripheral driving circuit in the present invention.
According to another feature of the invention, there is provided a monolithic active matrix circuit which is formed on a substrate, characterized in that a metal element is doped for promoting the crystallization of silicon at a concentration of 1xc3x971016 to 5xc3x971019 cmxe2x88x923 into at least a part of thin film transistors constituting a peripheral driving circuit in the present invention. The active area of the thin film transistor in the matrix area is constituted of an amorphous semiconductor film, and the active area of at least a part of the thin film transistors which constitute said peripheral circuit has a thin film silicon semiconductor film having a mono-domain structure.
In accordance with the present invention, a catalyst metal element is introduced into a selected portion of an amorphous silicon film selectively and a laser light irradiation is performed with the silicon film heated. Thus, a crystalline semiconductor film having a mono-domain structure is formed in the selected portion. This crystalline semiconductor film is suitable for forming an active region of a TFT for a driving circuit of an active matrix device because of its high mobility, high speed and large current characteristics. On the other hand, the other region of the silicon film, namely the region where the catalyst metal element is not introduced, remains amorphous. Accordingly, TFTs having active regions made of an amorphous silicon can be formed in a pixel region of the active matrix device on the same substrate, which do not have a high field mobility but a low off current property.
In this manner, a circuit having both low off current TFTs and high speed TFTs can be simultaneously formed on the same substrate.